Derrick's theorem [1]} offers a number of ways to fix the size
of the soliton.
One of these is the addition of a potential term and a fourth-order term in the
field derivatives to the Lagrangian of the nonlinear \(O\left( 3 \right)\) 
\(\sigma \) -model.
The modified nonlinear \(O\left(3\right)\)  \(\sigma \) -model is known as the baby
Skyrme model, and its topological solitons [2]}, [3]}, [4]}
(known as baby Skyrmions) have a fixed size.
The baby Skyrme model and other modifications to the nonlinear
\(O\left(3\right)\)  \(\sigma \) -model have applications in condensed matter physics
[5]}, [6]}, [7]}, [8]}.
This model also has applications in cosmology; it was shown in
Refs. [9]}, [10]}, [11]} that in the
six-dimensional Einstein-Skyrme model, the baby Skyrmion solutions realize
warped compactification of the extra dimensions and gravity localization on the
four-dimensional brane for the negative bulk cosmological constant.
